Several examples of this type of adapter are disclosed in Japanese Unexamined Patent Application Publication (JP-A) No. H06-82655. Each of those adapters comprises two adapter elements butted and coupled together and thus the number of components can be made small. The coupling between the adapter elements is performed by engaging means exposed on the outer surfaces of the adapter elements. The engaging means comprises a thin-plate elastic locking piece extending from the outer surface of each adapter element toward the mating adapter element and a recess formed on the outer surface of each adapter element. When the adapter elements are butted against each other, the elastic locking pieces are snap-engaged into the recesses, respectively, so that the adapter elements are coupled together. Thus, the assembly is easy.